The present invention relates to a circuit for demodulating a frequency modulated wave to reproduce an image signal, and more particularly to a circuit for preventing a lack of a carrier signal in the reproduced frequency modulated wave where the frequency modulated wave is recorded with a high modulation index.
In a magnetic image signal recording and reproducing apparatus such as a video tape recorder, a frequency modulated wave obtained by modulating a carrier with an image signal is recorded on a magnetic record medium such as a magnetic tape and the frequency modulated wave thus recorded is picked-up by a magneto-electric converting element such as a magnetic head to reproduce the frequency modulated wave. In such an apparatus, when the frequency modulated wave having a high modulation index is recorded, the following problem might occur.
In case of recording and reproducing a signal by means of a magnetic head having a finite gap width, an electromagnetic converting characteristic of a reproduced signal differs from an ideal characteristic curve a of 6 dB/octave and becomes curves b, c and d due to spacing loss, tape thickness loss and gap loss of the magnetic head, respectively. Therefore, the actual characteristic is dumped in a higher frequency range due to a total influence of these losses as shown by a curve e in FIG. 1.
In general, the image signal has a frequency band of several MHz and a carrier frequency to be modulated with the image signal should have a frequency higher than twice of the image signal frequency in order to avoid any interference between the carrier and the image signal. For this purpose, the carrier frequency, i.e. a fundamental frequency of the frequency modulated wave should usually be selected within a range of 8 to 10 MHz. Therefore, a frequency range F in which the reproduced level is reduced to a great extent has to be used in order to increase a utilization factor of the magnetic tape by effecting a high density recording.
FIG. 2 is a block diagram of a typical known magnetic video tape recording and reproducing apparatus. An image signal received at an input terminal 1 is supplied to an emphasis circuit 2 in order to improve a signal to noise ratio of a reproduced signal by emphasizing side-band components of the frequency modulated wave. After emphasizing higher frequency components in the circuit 2, the image signal is supplied to a frequency modulator 3 in which a carrier is frequency modulated with the image signal to produce a frequency modulated wave. The frequency modulated wave thus derived is supplied, through switch 4 actuated into a record side, to a magnetic head 5 and is recorded on a magnetic tape 6 which travels past the magnetic head 5 at a constant speed.
Upon reproduction, the switch 4 is actuated into a reproduction or play side and the frequency modulated wave recorded on the magnetic tape 6 is picked-up by the magnetic head 5 and the picked-up frequency modulated wave is supplied to an equalizer circuit 7. In the equalizer circuit 7, the reduction of high frequency component in the picked-up wave as shown by the curve e in FIG. 1 is compensated for. The frequency modulated wave thus compensated for is further supplied to a limiter circuit 8 to detect zero cross points of the reproduced frequency modulated wave. The detected zero cross signal representing only the frequency modulated component is supplied to a detector circuit 9 to multiply the frequency in order to prevent any interference between the frequency modulated wave and the image signal.
In a pulse count detection system, a pulse series having a constant pulse width is produced from the frequency multiplied signal and the pulse signal thus generated is supplied to a low pass filter 10 to remove the carrier component in the reproduced frequency modulated wave to derive as an output signal an average value of the pulse series. The output signal from the low pass filter 10 is further supplied to a de-emphasis circuit 11 for restoring the original high frequency component which has been emphasized by the emphasis circuit 2 during the recording. In this manner, a reproduced image signal having the same characteristic as that of the original image signal supplied to the input terminal 1 is derived at an output terminal 12.
Now it is assumed that the image signal has a frequency range up to 2 MHz and the fundamental frequency of the frequency modulated wave is set to 8 MHz, a frequency spectrum of the frequency modulated wave is as shown in FIG. 3. That is to say, a fundamental component J.sub.0 appears at 8 MHz and lower and upper side-band components such as -J.sub.1, -J.sub.2 and +J.sub.1 appear at lower and upper frequencies remote from the fundamental frequency by integer multiplies of 2 MHz. Usually in the magnetic recording and reproducing apparatus of a lower side-band system the upper side band components such as +J.sub.1 are not recorded and only the lower side-band components such as -J.sub.1, -J.sub.2 are recorded.
In the known magnetic recording and reproducing apparatus so far explained, there occurs a serious problem when the image signal changes abruptly. That is to say, when the image signal changes from a black peak level to a white peak level as illustrated in FIG. 4A, undesired noise called as an inverting phenomenon might be produced in the reproduced image signal. Now, the reason for generating such undesired phenomenon will be explained with reference to waveforms shown in FIGS. 4A to 4E.
In the recording process, the high frequency component of image signal shown in FIG. 4A is emphasized by the emphasis circuit 2 and thus, the image signal is converted into a differentiated wave as illustrated in FIG. 4B. Due to the principle of frequency modulation, the side-band components of the frequency modulated wave are emphasized and recorded on the magnetic tape 6. Therefore, upon the reproduction, the carrier frequency, i.e. the fundamental frequency is shifted toward a higher frequency at the emphasized side-band components and at the same time the side-band components -J.sub.1 and -J.sub.2 shown in FIG. 3 are reproduced with an emphasized magnitude. Further, in the magneto-electric converting characteristic, the high frequency component is dumped and the middle frequency component is increased as illustrated by the curve e in FIG. 1. Therefore, the side-band components -J.sub.1 and -J.sub.2 are further emphasized and thus, the frequency modulated wave shown in FIG. 4C is reproduced, in which the side-band components illustrated in FIG. 4D are superimposed upon the frequency modulated wave. The signal shown in FIG. 4C is derived from the equalizer circuit 7 and the side-band components are reduced to some extent by the frequency correcting action of the equalizer circuit 7. However, at the abrupt change of the image signal, the carrier component in the reproduced frequency modulated wave does not cross with a zero level L as shown by points P and P'. Then, the limiter circuit 8 does not produce an output signal or does produce a noise component which has been superimposed upon the reproduced frequency modulated wave. Therefore, in the reproduced image signal there are produced spike-like noises Q and Q' as illustrated in FIG. 4E at the abrupt change point of the original image signal. Such noises cause the so-called inverting phenomenon and there appears undesired noise in a reproduced picture.